1. Field of the Invention
The present invention relates to an electronic device comprising at least one support layer, a channel layer to contain an electron gas, a barrier layer and an ohmic contact electrode formed by a superposition of metallic layers the first of which is in contact with the barrier layer.
2. Description of Related Art
Electronic devices comprising at least one channel layer where an electron gas is able to flow, a barrier layer, and at least one ohmic contact electrode, find numerous applications.
Among these devices, field effect transistors of the High Electron Mobility Transistor (HEMT) type, or rectifiers are for example to be found. A HEMT presents two ohmic contact electrodes (called “source” and “drain”) and a Schottky contact electrode (called “gate”), whereas a rectifier comprises one ohmic contact electrode and one Schottky contact electrode.
Structures whose base is formed by group Ill/N materials are widely used in these applications, on account of their large bandgap.
The general structure of these devices will be described with reference to FIG. 1 which represents an example of a HEMT of known type. This description can apply to a rectifier considering the left-hand part of FIG. 1 only.
Such a device comprises a support layer 1 at its base, the role of which layer is essentially to provide the rigidity of the device. This support layer 1 is covered by a channel layer 3.
Optionally, a buffer layer 2 is placed between support layer 1 and channel layer 3. This buffer layer 2 presents a good crystallographic quality and suitable properties for growth by epitaxy of the other layers which cover it. It therefore ensures the crystallographic transition between support layer 1 and channel layer 3. The buffer layer further ensures that the voltage of the device is maintained. In the case of a transistor, the buffer layer is electrically insulating, as it has to improve confinement of the electrons in channel layer 3 reducing injection of charge carriers to the support. In the case of a rectifier, buffer layer 2 is given a particular conductivity so as to maintain the reverse voltage and guarantee forward conduction.
Channel layer 3 is an important layer as it enables circulation of an electron gas which can be two-dimensional and determines the performances of the component due to its electron transport properties.
The role of barrier layer 4 is to provide the structure with free electrons—it is the donor layer.
Ohmic contact electrode 5 enables carriers to be injected or recovered. In the case of a transistor, there are two ohmic contact electrodes—source 5 is the electrode that injects carriers into the structure, whereas drain 6 is the electrode which recovers the carriers. In the case of a rectifier, there is only one ohmic contact electrode 5. Ohmic contact electrode 5 is formed by a superposition of metallic layers deposited on superficial layer 7 or on the top surface of barrier layer 4 which in this case ensures a better ohmic contact.
Barrier layer 4 can be covered by a superficial layer 7 which prevents the structure from being damaged and contributes to ensuring a good Schottky contact with Schottky contact electrode 8 that is deposited there-above.
Lastly a passivation layer 9 encapsulates the device. In a general manner, passivation protects the surface of the semi-conductor.
To optimize the performances of electronic devices of this type, it is generally sought to improve the access resistance of the ohmic contact electrode. This resistance is directly linked to the resistivity of the superposition of metallic layers deposited to form the electrode and to the resistivity of the junction between this superposition of metallic layers and the semi-conducting material of the device.
It is therefore sought to optimize the contact between the metallic ohmic contact electrode and the semi-conducting material of the device, called ohmic contact, to obtain a reduced contact resistance, preferably lower than 1 Ω·mm. This contact resistance is the resistance of the material to the electric current flow and is measured using the Transmission Line Method (TLM). This method is described in detail in chapter III (“Contact resistance Schottky barrier and electromigration”) in the book “Semi-conductor material device characterization” by Dieter K. Schroder published by Wiley Publications.
To optimize the ohmic contacts, several methods are already known from the state of the art.
In the case where the ohmic contact electrodes are deposited on the superficial layer, the conductivity of the latter can be increased by doping. For example, if the superficial layer is made of GaN, doping is performed with n-type carriers such as Silicon and Germanium. But this type of doping modifies the electrical properties of the whole of the structure transforming its band energy diagram.
Another method consists in etching the superficial layer and the barrier layer until the channel layer is reached and in making a contact called “lateral contact” between the ohmic contact electrode and channel layer. But etching is tricky to perform as it may generate defects on the etching sides which will minimize the performances of the component (surface leakage current, reduction of breakdown voltage).
Implanting silicon in the barrier layer increases its conductivity with a contact resistance of 0.4 Ω·mm when the silicon content in the barrier layer reaches 1019 atoms per cm3 (reference can in this respect be made to the publication by S. Denbaars et al. in IEE vol. 26 N° 5 May 2005). However, to activate silicon by reorganization of the crystalline structure after implantation, very high temperature annealing is required (1500° C. under 100 bars of N2), and this type of method is difficult to apply on an industrial scale.
It is also possible to deposit a Titanium/Aluminium contact on the barrier layer and then perform annealing at 950° C. At this temperature, the metal diffuses in the barrier layer until it reaches the channel layer in which the electron gas is circulating and fosters a good ohmic contact. But this method comprises two drawbacks which are at the origin of leakage currents reducing the efficiency of the device: firstly the superficial layer has to be eliminated, and secondly the annealing temperature corresponds to that of formation of the material so that its surface is damaged during annealing. Deposition of an encapsulation layer (for example of AlN or Si3N4) to protect the surface during annealing is often necessary.
Finally, increasing the aluminium content from 20 to 30% in the AlGaN barrier layer considerably improves the conductivity of the layer, but the AlGaN alloy can thereby lose its homogeneity and become unstable. The strain stored in the AlGaN layer is in fact the greater the higher the aluminium content, and relaxation of this strain is moreover frequently observed by premature aging of the devices. Moreover, too high an aluminium content in the barrier layer results in an increase of gate leakage currents. Embodiments disclosed herein address these issues.